In radiofrequency (“RF”) therapeutic systems, all monopolar therapeutic energy produced by a RF generator should theoretically return to the generator via a patient return electrode. The therapeutic path typically flows from the RF generator to an active accessory, to a target, i.e., a patient, to a return electrode and back to the RF generator. However, due to the capacitance of transformers in isolation barriers which serve to isolate the supply of RF energy between the RF generator and the delivery device, sometimes stray leakage in the form of RF energy flows from the RF generator to ground instead of returning to the RF generator as part of the therapeutic path. RF leakage is a cause of concern to users because of the dangerous amount of current that can enter the patient, the surgeon or other medical personal that are in contact with the patient.
RF generator designers face the challenge of ensuring that the connection between the patient and the equipment in the RF therapeutic system minimizes leakage current under both normal system operation and under fault conditions. Designers are further faced with the challenge of meeting the isolation and leakage current requirements of Standard IEC60601, which defines the safety and electromagnetic compliance (“EMC”) for medical systems.
Another challenge facing RF generator designers is the design of generators that comply with Standard IEC60601, particularly the standard that requires that the system maintains safe operation during any single fault condition and does not exceed the power output limits set forth by the IEC 60601 standard. While software exists that controls the output power based on feedback, other effective non-software implementations are required since a software failure alone cannot allow dangerous conditions to exist.
Typical RF generators have outputs that are isolated from ground in order to prevent RF leakage. However, isolated output circuits are, by themselves, not enough to completely eliminate RF leakage. Some RF generators have been designed with the capability of detecting open circuits and being able to lower their peak output voltage accordingly. This leads to several performance problems. Lowering peak output voltage in order to minimize leakage current may degrade the performance of the RF generator since the peak output voltage initiates the sparking needed for proper coagulation effect. Further, the amount of time needed by the generator to sense the open circuit condition may lead to momentary voltage spikes, which could cause RF leakage to occur.
Other types of predicate generators introduce throttling schemes when unacceptable leakage levels are detected. These throttling schemes have proven to be inadequate because they add undesired complexity to the control system.
FIG. 1 illustrates a circuit diagram of a typical feedback estimation system for use in an RF therapeutic system. Circuit 10 includes a direct current (“DC”) power supply 11 that generates DC voltage which is passed into a voltage buck regulator by PWM control of a transistor 12. The PWM output waveform from transistor 12 is filtered through a filter 14 to create a reduced DC voltage that passes through an H-bridge RF wave generator 16 to be transformed into RF energy in the form of an RF waveform which is applied to an electrosurgical instrument (not shown), which is used to treat the target patient 18.
Continuing with the prior art circuity illustrated in FIG. 1, the RF output circuit may include a high-turns transformer 22 across an isolation barrier 17, which serves to isolate a supply of RF energy from the electrosurgical instrument. A voltage sensor 28, which includes a voltage divider 20 and a high-turns transformer with couplings 24, measures RMS output voltage feedback at location (1) in circuit 10. Current sensor 30 also includes a transformer with couplings 26 and is configured to measure RMS output current feedback at location (2) in circuit 10. Thus, circuit 10 includes three inductive couplings, 22, 24 and 26, across isolation barrier 17. Based on the AC RMS output voltage feedback and the RMS output current feedback, a microprocessor 32 calculates an estimated output voltage and power and adjusts a control output Pulse Width Modulation (“PWM”) signal output at location (4) to control the voltage input to H-bridge RF wave generator 16 and maintain its output at desired levels. However, the large number of inductance couplings in isolation barrier 17 result in excess capacitance and ultimately unduly high levels of leakage current. Therefore, a different RF feedback estimation circuit is desired.